1. Field of the Invention
The present invent ion relates to a semiconductor device whose active region is formed from a semiconductor film constituted of a mass of crystals with various orientations (the film hereinafter is referred to as crystalline semiconductor film). Typical example of the crystalline semiconductor film is a polycrystalline silicon film. Specifically, the invention relates to a thin film transistor or a semiconductor device that has a circuit composed of the thin film transistor. The term semiconductor device herein refers to a device in general which utilizes semiconductor characteristics to function, and semiconductor integrated circuits, electro-optical devices and electronic equipment fall within this category.
2. Description of the Related Art
A technique has been developed to manufacture a thin film transistor (hereinafter referred to as TFT) from a crystalline semiconductor film with a thickness of several nm to several hundreds nm. TFTs are now established as practical switching elements for liquid crystal display devices, which has brought the recent success in forming a semiconductor integrated circuit on a glass substrate.
Silicon is a material of the crystalline semiconductor film that is suitable for a TFT. Used as this crystalline semiconductor film is a silicon film having a crystal structure (hereinafter referred to as crystalline silicon film). The crystalline silicon film is obtained by forming an amorphous silicon film on a glass or quarts substrate through deposition by plasma CVD or reduced pressure CVD and crystallizing the amorphous silicon film through heat treatment or laser light irradiation (will be called laser treatment in this specification).
When heat treatment is chosen, the amorphous silicon film has to be heated at a temperature of 600xc2x0 C. or higher for 10 hours or longer to crystallize. Considering the productivity in manufacturing TFTs, it is difficult to say the method with the treatment temperature this high and the treatment time this long is a proper method. Taking a liquid crystal display device as an example of a product to which the TFTs are applied, a large-sized heat treatment furnace is required in order to accommodate the substrate as its surface area becomes larger. This not only increases energy consumption in manufacturing process but also makes it difficult to obtain uniform crystals over the large surface area. On the other hand, when laser treatment is chosen, obtaining crystals of uniform quality is still difficult because the output of a laser oscillator is not stable. The diversity in quality between crystals results in fluctuation in characteristic between TFTs, which in turn causes lowering of display quality of the liquid crystal display device or a display device whose pixel portion is composed of light emitting elements.
Another technique has been disclosed in which a metal element for promoting crystallization of silicon is introduced in an amorphous silicon film so that a crystalline silicon film is formed by heat treatment at a temperature lower than in the conventional heat treatment. For example, Japanese Patent Application Laid-open Nos. Hei 7-130652 and Hei 8-78329 show that a crystalline silicon film can be obtained by introducing a metal element such as nickel into an amorphous silicon film and heating the film at 550xc2x0 C. for four hours.
However, a TFT manufactured by using the thus formed crystalline silicon film is still inferior in characteristics to a MOS transistor comprised of a single crystal silicon substrate. If a semiconductor film with a thickness of several nm to several hundreds nm is subjected to crystallization process on a material different from the film, such as glass or quartz, only a polycrystalline structure composed of masses of plural crystal grains is obtained. In the polycrystalline structure, carriers are trapped by an infinite number of defects found in crystal grains and in grain boundaries to limit the performance of the TFT.
In the crystalline silicon film formed by the above method of prior art, crystal orientation planes are arranged at random and the orientation ratio of a specific crystal orientation is low. The crystalline silicon film obtained by heat treatment or laser treatment has plural crystal grains deposited and tends to orient in {111} orientation, although the ratio of that part that is oriented to the {111} plane to the entire film does not exceed 20%.
When the orientation ratio is low, it is nearly impossible to keep the continuity of lattice in the grain boundaries where crystals of different orientations meet, and hence many dangling bonds will presumably be generated. The dangling bonds generated in the grain boundaries work as trap centers for carriers (electrons and holes) to degrade the carrier transportation characteristic. To elaborate, carriers are scattered or trapped in such film and the crystalline semiconductor film with scattered or trapped carriers is not expected to turn into a TFT that is high in field effect mobility. Furthermore, grain boundaries are arranged at random, meaning that a channel formation region cannot be formed from crystal grains of a specific crystal orientation. This can cause fluctuation in electric characteristics of TFTs.
The present invention has been made to solve the above problems and an object of the present invention is to improve the orientation of a crystalline semiconductor film obtained by crystallizing an amorphous semiconductor film and to provide a TFT formed from the crystalline semiconductor film.
In order to solve the above problems, according to a structure of the present invention, there is provided a semiconductor device having a thin film transistor formed of a crystalline semiconductor film that contains silicon as its main ingredient and germanium, characterized in that:
the crystalline semiconductor film has a channel formation region and an impurity region that is doped with an impurity of one type of conductivity;
20% or more of the channel formation region is the {101} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film, the plane being detected by an electron backscatter diffraction pattern method;
3% or less of the channel formation region is the {001} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film;
5% or less of the channel formation region is the {111} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film; and
secondary ion mass spectroscopy is conducted on the channel formation region to reveal that the region contains less than 5xc3x971018 nitrogen atoms per cm3, less than 5xc3x971018 carbon atoms per cm3, and less than 1xc3x971019 oxygen atoms per cm3.
Further, according to another structure of the present invention, there is provided a semiconductor device having a thin film transistor formed by doping an amorphous semiconductor film with a metal element and by subjecting it to heat treatment and laser treatment, the amorphous semiconductor film containing silicon as its main ingredient and germanium, characterized in that:
the crystalline semiconductor film has a channel formation region and an impurity region that is doped with an impurity of one type of conductivity;
20% or more of the channel formation region is the {101} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film, the plane being detected by an electron backscatter diffraction pattern method;
3% or less of the channel formation region is the {001} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film;
5% or less of the channel formation region is the {111} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film; and
secondary ion mass spectroscopy is conducted on the channel formation region to reveal that the region contains less than 5xc3x971018 nitrogen atoms per cm3, less than 5xc3x971018 carbon atoms per cm3, and less than 1xc3x971019 oxygen atoms per cm3.
Further, according to still another structure of the present invention, there is provided a semiconductor device whose pixel portion and driver circuit are formed on the same insulator, characterized in that:
thin film transistors in the pixel portion and in the driver circuit are all n-channel transistors;
each of the thin film transistors has a channel formation region formed of a crystalline semiconductor film that contains silicon as its main ingredient and germanium;
20% or more of the crystalline semiconductor film is the {101} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film, the plane being detected by an electron backscatter diffraction pattern method;
3% or less of the crystalline semiconductor film is the {001} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film;
5% or less of the crystalline semiconductor film is the { 111) lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film; and
secondary ion mass spectroscopy is conducted on the crystalline semiconductor film to reveal that the film contains less than 5xc3x971018 nitrogen atoms per cm3, less than 5xc3x971018 carbon atoms per cm3, and less than 1xc3x971019 oxygen atoms per cm3.
Moreover, according to still another structure of the present invention, there is provided a semiconductor device whose pixel portion and driver circuit are formed on the same insulator, characterized in that:
thin film transistors in the pixel portion and in the driver circuit are all p-channel transistors;
each of the thin film transistors has a channel formation region formed of a crystalline semiconductor film that contains silicon as its main ingredient and germanium;
20% or more of the crystalline semiconductor film is the {101} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film, the plane being detected by an electron backscatter diffraction pattern method;
3% or less of the crystalline semiconductor film is the {001} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film;
5% or less of the crystalline semiconductor film is the {111} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film; and
secondary ion mass spectroscopy is conducted on the crystalline semiconductor film to reveal that the film contains less than 5xc3x971018 nitrogen atoms per cm3, less than 5xc3x971018 carbon atoms per cm3, and less than 1xc3x971019 oxygen atoms per cm3.
Further, according to still another structure of the present invention, there is provided a semiconductor device whose pixel portion and driver circuit are formed on the same insulator, characterized in that:
the driver circuit is composed of an n-channel thin film transistor and a p-channel thin film transistor;
each of the n-channel and p-channel thin film transistors has a channel formation region formed of a crystalline semiconductor film that contains silicon as its main ingredient and germanium;
20% or more of the crystalline semiconductor film is the {101} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film, the plane being detected by an electron backscatter diffraction pattern method;
3% or less of the crystalline semiconductor film is the {001} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film;
5% or less of the crystalline semiconductor film is the 111} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film; and
secondary ion mass spectroscopy is conducted on the crystalline semiconductor film to reveal that the film contains less than 5xc3x971018 nitrogen atoms per cm3, less than 5xc3x971018 carbon atoms per cm3, and less than 1xc3x971019 oxygen atoms per cm3.
Furthermore, according to still another structure of the present invention, there is provided a semiconductor device whose pixel portion is formed on an insulator, characterized in that:
thin film transistors in the pixel portion each have a channel formation region formed of a crystalline semiconductor film that contains silicon as its main ingredient and germanium;
20% or more of the channel formation region is the {101} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film, the plane being detected by an electron backscatter diffraction pattern method;
3% or less of the channel formation region is the {001} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film;
5% or less of the channel formation region is the {111} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film; and
secondary ion mass spectroscopy is conducted on the channel formation region to reveal that the region contains less than 5xc3x971018 nitrogen atoms per cm3, less than 5xc3x971018 carbon atoms per cm3, and less than 1xc3x971019 oxygen atoms per cm3.
Besides, according to still another structure of the present invention, there is provided a semiconductor device whose pixel portion and driver circuit are formed on the same insulator, characterized in that:
the driver circuit includes a buffer composed of thin film transistors of one type of conductivity;
the buffer has a first one conductivity thin film transistor and a second one conductivity thin film transistor, the second one conductivity thin film transistor being connected to the first one conductivity thin film transistor in series and having as its gate a drain of the first one conductivity thin film transistor;
each of the first and second thin film transistors has a channel formation region formed of a crystalline semiconductor film that contains silicon as its main ingredient and germanium;
20% or more of the crystalline semiconductor film is the {101} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of the crystalline semiconductor film, the plane being detected by an electron backscatter diffraction pattern method;
3% or less of the crystalline semiconductor film is the {001} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of crystalline semiconductor film;
5% or less of the crystalline semiconductor film is the {111} lattice plane that forms an angle of equal to or less than 10 degree with respect to the surface of crystalline semiconductor film; and
secondary ion mass spectroscopy is conducted on the crystalline semiconductor film to reveal that the film contains less than 5xc3x971018 nitrogen atoms per cm3, less than 5xc3x971018 carbon atoms per cm3, and less than 1xc3x971019 oxygen atoms per cm3.
The crystalline semiconductor film used in the present invention is obtained by doping an amorphous semiconductor film that contains silicon as its main ingredient and germanium with a metal element and crystallizing the film through heat treatment, or through heat treatment plus laser light irradiation. The metal element to be used for the doping is one or more elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au. By doping the amorphous semiconductor film with the metal element(s) given above and then subjecting the film to heat treatment, a compound of silicon and the metal element(s) (silicide) is formed. The silicide diffuses throughout the film to advance crystallization.
At this point, germanium does not react with this compound but causes local distortion by merely existing around the compound. The distortion increases the critical radius of nuclear generation and, overall, acts to reduce the nuclear generation density. The distortion also works to limit orientation of crystals.
The concentration of germanium suitable for inducing such effects is found to be 0.1 atomic percent or more and 10 atomic percent or less, preferably 1 atomic percent or more and 5 atomic percent or less, as a result of experiments. When the concentration of germanium is higher than the above, natural nuclei as alloy materials of silicon and germanium are generated in a considerable number, making it impossible to raise the orientation ratio. (Natural nuclei are crystal nuclei generated not from the compound of silicon and the dopant metal element but from other compounds.) The germanium concentration lower than the above also cannot raise the orientation ratio because the distortions generated are not enough.
When an amorphous semiconductor film is crystallized, atoms are rearranged so that the volume of the film is reduced macroscopically. As a result, tensile stress is generated in the crystalline semiconductor film formed on the substrate. However, if the amorphous semiconductor film is doped with 0.1 to 10 atomic percent, preferably 1 to 5 atomic percent, of germanium that has an atomic radius larger than that of silicon, the volume shrinkage accompanying crystallization is reduced and the tensile stress to be generated is accordingly reduced. In short, doping with germanium in a given concentration can ease distortion in the crystalline semiconductor film.
The distribution of crystal orientation is obtained by electron backscatter diffraction pattern (hereinafter abbreviated as EBSP). EBSP is a method of analyzing the crystal orientation from backscatter of the primary electron by setting a dedicated detector in a scanning electron microscope. The measurement method by EBSP is illustrated in FIG. 6. An electron gun (Schottky type field emission electron gun) 101, a mirror 102 and a sample chamber 103 are structured in the same way as those in an ordinary scanning electron microscope. In EBSP measurement, a stage 104 is slanted about sixty degree and a sample 109 is placed thereon. A screen 105 of a detector 106 is inserted so as to face the sample in this state.
If an electron beam enters the sample having a crystal structure here, inelastic scattering takes place also in the rear. There can also be observed a linear pattern peculiar to the crystal orientation by Bragg diffraction in the sample (the pattern is commonly called a Kikuchi image). EBSP obtains the crystal orientation of the sample by analyzing the Kikuchi image projected onto the screen of the detector.
FIG. 7 shows a crystalline semiconductor film 122 having a polycrystalline structure and formed on a substrate 121. Having a polycrystalline structure means that crystal grains have crystal orientations different from one another. Information of the crystal orientation or orientation can be obtained for a planar sample by the mapping measurement in which the point the electron beam hits the sample is moved along and the orientation is analyzed every time the point moves. The thickness of the incident electron beam varies depending on the type of the electron gun attached to the scanning electron microscope. In the case of the Schottky field discharge type, the gun emits a very thin electron beam 123 with a diameter of 10 to 20 nm. The mapping measurement can provide more averaged information of the crystal orientation when the number of measurement points is greater and the area of the measurement range is wider. In a practical measurement, an area of 100xc3x97100 xcexcm2 is measured at about 10000 points (the distance between two points is 1 xcexcm) to 40000 points (the distance between two points is 0.5 xcexcm).
When the crystal orientation is obtained for all of the crystal grains from the mapping measurement, the crystal orientation state relative to the film can be expressed statistically. FIG. 8A shows an example of reverse pole diagram obtained by EBSP. A reverse pole diagram is often used to show the major orientation of a polycrystal, and it collectively illustrates correspondence between a specific face of the sample (here, the film surface) and a lattice plane.
The fan-shaped frame in FIG. 8A is the one generally called a standard triangle in which all indexes related to the cubic system are included. In FIG. 8A, the length corresponds to the angle in the crystal orientation. For instance, the distance between {001} and {101} is 45xc2x0, the distance between {101} and {111} is 35.26xc2x0, and the distance between {111} and {001} is 54.74xc2x0. The white dotted lines respectively indicate a range of offset angle of 5xc2x0 and a range of offset angle of 10xc2x0 relative to {101}.
FIG. 8A is obtained by plotting all of the measurement points (11655 points in this case) in the mapping measurement onto the standard triangle. The points are dense in the vicinity of {101}. FIG. 8B translates concentration of points of FIG. 8A into contour. This is an orientation distribution function values for expressing as contour the concentration (the density of the points in FIG. 8A) premised on random orientation. The numeric values here are scale factors (magnification) when assuming that the orientation of crystal grains is completely random, namely, when the points are evenly distributed throughout the standard triangle, and the values are dimensionless numbers.
If it is found that there is the major orientation toward a specific index (here, {101}), the level of the major orientation is easy to image when the quantity of crystal grains centered around the specific index is expressed in numeric values as above. For example, the orientation ratio is expressed by and obtained from the following equation when the orientation ratio is given as the ratio of the points present in the range of offset angle of 5xc2x0 and the range of offset angle of 10xc2x0 relative to {101} to the whole points in the reverse pole diagram of FIG. 8A shown as an example (the ranges are indicated by the white dotted lines in FIG. 8A).
{101} orientation ratio=(the number of the measured points within acceptable offset angle formed between {101} lattice plane and film surface)/(the number of the measured whole points)xe2x80x83xe2x80x83Equation 1
Alternatively, this ratio can be described as follows. When the points are distributed heavily around {101} as in FIG. 8A, it is expected in the actual film that the {101} orientation of the grains is substantially perpendicular to the substrate although there are some fluctuation in orientation as shown in FIG. 10. The acceptable error for the fluctuation angle is 5xc2x0 and 10xc2x0. Then the number of crystal grains whose {101} orientation is smaller than the acceptable angle is counted to express the ratio of them in numeric values. For example, the {101} orientation of a certain crystal grain in FIG. 9 is not in the acceptable range of 5xc2x0 but in the acceptable range of 10xc2x0. In obtaining data later in this specification, the acceptable offset angles are set to 5xc2x0 and 10xc2x0 and the ratio of crystal grains that fall within the acceptable ranges are calculated as described above.
In the reverse pole diagram shown as an example in FIG. 8A, the peaks respectively represent {101}, {111} and {001}, and the diagram shows that other plane orientations emerge when the offset angle with respect to {101} exceeds certain values. For example, the {112} orientation emerges when the offset angle with respect to {101} reaches 30xc2x0. Accordingly, when EBSP is used to determine the ratio of crystal orientations, it is necessary to set an acceptable offset angle for crystal grains that are distributed with fluctuation to such an angle as to exclude any possibility of erroneously counting other indexes in. Experientially, appropriate acceptable offset angle is 10xc2x0 or less, or 5xc2x0 or less. When data is collected with the acceptable offset angle set to the angle above, the ratio of crystal grains oriented in a specific orientation can be quantified.